XPath Notes

XPath Introduction

XPath is a set of syntax rules for defining parts of an XML document.

XPath uses paths to define XML elements.

XPath defines a library of standard functions.

XPath is a major element of XSLT.

XPath is not written in XML.

XPath is a W3C standard.
- Like Traditional File Paths
  
  XPath uses path expressions to identify nodes in an XML document.
  
  These path expressions look very much like regular file system path names.
- Example XML Document:
```
              <?xml version="1.0" encoding="ISO-8859-1"?>
              <catalog>
                <cd country="USA">
                  <title>Empire Burlesque</title>
                  <artist>Bob Dylan</artist>
                  <price>10.90</price>
                </cd>
                <cd country="UK">
                  <title>Hide your heart</title>
                  <artist>Bonnie Tyler</artist>
                  <price>9.90</price>
                </cd>
                <cd country="USA">
                  <title>Greatest Hits</title>
                  <artist>Dolly Parton</artist>
                  <price>9.90</price>
                </cd>
              </catalog>
            
```
  The XPath expression below selects the root element "catalog".
```
              /catalog
            
```
  The XPath expression below selects all the "cd" elements of the "catalog" element.
```
              /catalog/cd
            
```
  The XPath expression below selects all the "price" elements of all the "cd" elements of the "catalog" element.
```
              /catalog/cd/price
            
```
  If the path starts with a "/", it represents an absolute path to an element.
- XPath Defines a Library of Standard Functions
  
  For working with strings, numbers and boolean expressions.
  
  The XPath expression below selects all the "cd" elements that have a price element with a value greater than 10.80.
```
              /catalog/cd[price>10.80]
            
```
- XPath is Used in XSLT.
  
  XPath is a major element of the XSLT standard.
  
  XPath was designed to be used by XSLT, XPointer and other XML parsing software.
XPath Syntax
XPath uses Path Expressions to locate Nodes within XML documents.
- Example XML Document:
```
              <?xml version="1.0" encoding="ISO-8859-1"?>
              <catalog>
                <cd country="USA">
                  <title>Empire Burlesque</title>
                  <artist>Bob Dylan</artist>
                  <price>10.90</price>
                </cd>
                <cd country="UK">
                  <title>Hide your heart</title>
                  <artist>Bonnie Tyler</artist>
                  <price>9.90</price>
                </cd>
                <cd country="USA">
                  <title>Greatest Hits</title>
                  <artist>Dolly Parton</artist>
                  <price>9.90</price>
                </cd>
              </catalog>
            
```
- Locating Nodes
  XML documents can be represented as a tree view of nodes similar to a view of file folders
  
  XPath uses a pattern expression to identify nodes in an XML document.
  
  An XPath pattern is a slash separated list of child element names that describe a path through the XML document.
  
  The pattern selects elements that match the path.
  
  The following XPath expression selects all the price elements of all the cd elements of the catalog element in the example XML document.
```
                /catalog/cd/price
              
```
  If the path starts with "/" it represents an absolute path to an element.
  
  If the path starts with "//" then all the elements in the document that fulfill the criteria will be selected even if they are at different levels in the XML tree.
  
  The following XPath expression selects all the "cd" elements in the document.
```
                //cd
              
```
- Selecting Unknown Elemenets
  Wildcards, *, can be used to select unknown XML elements.
  
  The following XPath expression selects all the child elements of all the "cd" elements of the "catalog" element.
```
                /catalog/cd/*
              
```
  The following XPath expression selects all the grandchild elements of the "catalog" element
```
                /catalog/*/price
              
```
  The following XPath expression selects all "price" elements that have 2 ancestors.
```
                /*/*/price
              
```
  The following XPath expression selects all the elements in the document
```
                //*
              
```
- Selecting Branches
  By using square brackets in an XPath expression you can specify and element futher
  
  The following selects the first "cd" element of the "catalog" element
```
                /catalog/cd[1]
              
```
  The following selects the last child element of the "catalog" element. Note: there is no function called first()
```
                /catalog/cd[last()]
              
```
  The following selects all the "cd" elements of the catalog "element" that have a "price" element
```
                /catalog/cd[price]
              
```
  The following selects all the "cd" elements of the "catalog" element that have a "price" element with a value of 10.90.
```
                /catalog/cd[price = 10.90]
              
```
  The following selects all the "price" elements of all the "cd" elements of the "catalog" element that have a "price" element with a value of 10.90
```
                /catalog/cd[price = 10.90]/price
              
```
- Selecting Several Paths
  By using the | operator you can select several paths.
  
  The following selects all the"title" and "artist" elements of the "cd" element of the "catalog" element.
```
                /catalog/cd/title | /catalog/cd/artist
              
```
  The following selects all the title and artist elements in the document
```
                //title | //artist
              
```
  The following selects all the title, artist, and price elements
```
                //title | //artist | //price
              
```
  The following selects all teh title elements of the cd element of the catalog element and all the artist elements in the document
```
                /catalog/cd/title | //artist
              
```
- Selecting Attributes
  In XPath attributes are specified by the @ prefix.
  
  The following selects all attributes named country.
```
                //@country
              
```
  The following selects all cd elements with an attribute named country
```
                //cd[@country]
              
```
  The following selects all cd elements that have any attribute
```
                //cd[@*]
              
```
  The following selects all cd elements that have an attribute named country which have a value of 'UK'.
```
                //cd[@country = 'UK']
              
```

XPath Location Paths

A location path expression results in a node-set.

Location Path Expression
A location path can be absolute or relative.

An absolute location path starts with "/" and a relative locaion path does not.

In both cases the location path consists of one or more location steps, each separated by "/".

Absolute Location Path Example:
```
                /step/step/...
              
```
Relative Location Path Example:
```
                step/step/...
              
```
The location steps are evaluated in order one at a time from left to right.

Each step is evaluated against the nodes in the current node set.

If the location path is absolute, the current node set consists of the root node.

If the location path is relative, the current node set consists of the node where the expression is being used.

Location Steps Consist Of
An axis which specifies the tree relationship between the nodes selected by the location step and the current node.

A node test which specifies the node type and expanded name of the nodes selected by the location step.

Zero or more predicates which use expressions to further refine the set of nodes selected by the location step.

The syntax for a location step is:
```
                    axisname::nodetest[predicate]
                  
```
Example:
```
                    child::price[price=9.90]
                  
```

Axes and Node Tests

An axis defines a node set relative to the current node.

A node test is used to identify a node within an axis and can be performed by name or by type.

AxisName	Description
ancestor	Contains all ancestors of the current node This axis will always include the root node unless the current node is the root node.
ancestor-or-self	Contains the current node plus all it's ancestors
attribute	Contains all attributes of the current node
child	contains all children of the current node
descendant	contains all descendants of the current node This axis never contains attribute or namespace nodes
descendant-or-self	Constains the current node plus al it's descendants
following	Contains everything in the document after the closing tag of the current node
following-sibling	Contains all siblings after the current node. If the current node is an attribute node or namespace node this axis will be empty
namespace	Contains all namespace nodes of the current node
parent	Contains the prent of the current node
preceding	Contains everything in the document that is before the starting tag of the current node
preceding-sibling	Contains all siblings before the current node If the current node is an attribute node or namespace node this axis will be empty
self	Contains the current node

Examples

Example	Result
child::cd	Selects all cd elements that are children of the current node If the current node has no cd children, it will select an empty node-set
attribe::src	Select the src attribute of the current node If the current node has no src attribute, it will select an empty node-set
child::*	Selects all child element of the current node
attribute::*	Selects all attributes of the current node
child::text()	Selects the text node children of the current node
child::node()	Selects all the chidren of the current node
descendant::cd	Selects all the cd element descendants of the current node
ancestor::cd	Selects all cd ancestors of the current node
ancestor-or-self::cd	Selects all cd ancestors of the current node and, if the current node is a cd element, the current node as well
child::&/child::price	Selects all price grandchildren of the current node
/	Selects the docuemnt root

Predicates

A predicate filters a node-set into a new node-set.

A predicate is placed inside [].

Example	Result
child::price[price=9.90]	Selects all prices elements that are children of the current node with a price element = 9.90
child::cd[position()=1	Selects the first cd child of the current node
child::cd[position()=last]	Selects the last cd child of the current node
child::cd[position()=last()-1	Selets the last but one cd child of the current node
child::cd[position()<6]	Selects the first 5 cd children of the current node
/descendant:;cd[position()-7]	Selects the 7th cd element in the document
child::cd[attribute::type="classic"]	Selects all cd children of the current node having a type attribute with the value of "classic"

Location Path Abbreviated Syntax

Abbreviations can be used when describing a location path.

The most important abbreviation is that child:: can be omitted from a location step.

Abbr	Meaning	Example
name	child::	cd is short for child::cd
@	attribute::	cd[@type="classic"] is short for child::cd[attribute::type="classic"]
.	self::node()	.//cd is short for self::node()/descendant-orself::node()/child::cd
..	parent::node()	..//is short for parent::node()/child::cd
//	/descendant-or-self::node()/	//cd is short for /descendant-or-self::node()/child::cd

Examples

Example	Result
cd	Selects all the cd elements that are children of the current node
*	Selects all child elements of the current node
text()	Selects all text node children of the current node
@src	Selects the src attribute of the current node
@*	Selects all attributes of the current node
cd[1]	Selects the first cd child of the current node
cd[last()]	Select the last cd child of the current node
*/cd	Selects all cd grandchildren of the current node
/book/chaper[3]/para[1]	Selects the first para of the third chapter of the book
//cd	Selects all the cd descendants of the document root, and thus selects all cd elements in the same document as the current node
.	Selects the current node
.//cd	Selects the cd element descendants of the current node
..	Selects the parent of the current node
../@src	Selects the src attribute of the parent of the current node
cd[@type="classic"]	Selects all cd children of the current node that have a type attribute with the value "classic"
cd[@type="classic"][5]	Selects the 5th cd child of the current node that has a type attribute with the value "classic"
cd[5][@type="classic"]	Selects the 5th cd child of the current node if that child has a type attribute with the value "classic"
cd[@type and @country]	Selects all teh cd children of the current node that have both a type attribute and a country attribute

XPath Expressions

XPath supports numeric, equality, relational and boolean expressions

Numeric Expressions

Used to perform arithmetic operations on numbers.

Operator	Description	Example	Result
+	Addition	6 + 4	10
-	Subtraction	6 - 4	2
*	Multiplication	6 * 4	24
div	Division	8 div 4	2
mod	Modulus	5 mod 2	1

XPath always converts each operand to a number before performing the operation

Equality Expressions

Operator	Description	Example	Result
=	Equal	price=9.90	true if price = 9.90
!=	Not equal	price!=9.90	false

Testing Against a Node-Set.

If the test value is tested for equality against a node-set, the result is true if the node-set contains any node with a value that matches the test value.

If the test value is tested for not equal against a node-set, the result is true if the node-set contains any node set with a value that differs from the test value.

Therefore the node-set can be equal and not equal at the same time.

Relational Expressions

Operator	Description	Example	Result
<	Less Than	price<9.90	false if price is 9.90
<=	Less Than or Equal to	price<=9.90	true
>	Greater Than	price>9.90	false
>=	Greater Than or Equal To	price>=9.90	True

XPath always converts each operand to a number before performing the evaluation

Boolean Expressions

Used to compare 2 values

Operator	Description	Example	Result
or	or	price=9.90 or price=9.70	true if price = 9.90
and	and	price<=9.90 and price=9.70	false

XPath Functions for Converting/Translating XML Data

Node Set Functions

Name	Description	Syntax & Example
count()	Returns the number of nodes in a node-set	number=count(node-set)
id()	Selects elements by their Id	node-set=id(value)
last()	Returns the position number of the last node in the processed node list	number=last()
local-name()	Returns the local part of a node. A node usually consists of a prefix, a colon, followed by the local name	string=local-name(node)
name()	Returns the name of a node	string=name(node)
namespace-uri()	Returns the namespace URI of a node	uri=namespace-uri(node)
position()	Returns the position in the node list of the current node	number=position()

String Functions

Name	Description	Syntax & Example
concat()	Returns the concatenation of all it's arguements	string-contcat(value1,value2,...)
contains()	Returns true if the second string is contained within the first	bool=contains(string,substring)
naormalize-space()	Removes leading/trailing spaces	string=normalize-space(string)
starts-with()	Returns true if the first string starts with the second	bool=starts-with(string,substr)
string()	Converts the value to a string	string(value)
string-length()	Returns the length	number=string-length(string)
substring()	Returns a part of the string	string=substring(string,start,length)
substring-after()	Returns everyting after the substring	string=substing-after(string,substr)
substring-before	Returns everything before the substring	string=substring-before(string,substr)
translate()	Performs a char by char replacement like a replace	string=translate(string,substring1,substring2

Number Functions

Name	Description	Syntax & Example
ceiling()	Returns the smallest integer that is not less than the number argument	number=ceiling(number)
floor()	REturns the largest integer that is not greater than the number argument	number=floor(number)
number()	Converts a value to a number	number=number(value)
round()		integer=round(number)
sum()		number=sum(nodeset)

Boolean Functions

Name	Description	Syntax & Example
boolean()	Converts to a boolean	bool=boolean(value)
false()	Returns false
lang()	Returns true if the language argument matches the language of the sl:lang element	bool=lang(language)
not()	Returns true if the conditions is false	bool=not(condition)
true()	Returns true

XPath Examples

Examples are against the CD catalog

Selecting Nodes from an XML document

              xmlobject.selectNodes(XPath expression)

The following selects all cd nodes from the CD catalog

              xmlDoc.selectNodes("/catalog/cd")

Selecting the first cd Node
The following example selects only the first cd node from the CD catalog.
```
              xmlDoc.selectNodes("/catalog/cd[0]")
            
```

Selecting Price Nodes

The following selects all teh price nodes from the Cd catalog

              xmlDoc.selectNodes("/catalog/cd/price")

Selecting Price Text Nodes

The following selects only the text from teh price nodes.

              xmlDoc.selectNodes("/catalog/cd/price/text()")

Selecting cd Nodes with Price>10.00

The following selects all teh cd nodes with a price>10.00

              xmlDoc.selectNodes("/catalog/cd[price>10.00]")

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